Traditionally physicists thought that the smallest particles in atoms were point like, that is, they looked like points. But in 1968 a formula describing one of the forces was discovered; however it did not represent a point particle, it represented a vibrating string. This suggested that the smallest particle is not point like, as previously thought, but rather a vibrating string of energy.
But physicists also discovered that these strings need more than
three spatial dimensions to vibrate. This discovery lead to new theories with extra
spatial dimensions. However the physics in those extra
dimensions might not be the same as ours, effectively forming parallel universes. Those parallel universes can only be detected by their gravity. The strongest evidence for parallel universes is Dark
Matter. We cannot see nor collide with this Dark
Matter however we can detect its gravity. The largest discovered
cluster of Dark Matter has gravity equivalent to 10,000 galaxies
(a quadrillion times that of the Sun, 1015!!!). In
String Theory this
Dark Matter is mass in other dimensions. However in M-Theory this Dark
Matter is mass in parallel universes (the laws of physics in
those parallel universes might be different than ours).
Those
videos will introduce you to String and M-Theory:
